Background Field
Embodiments of the subject matter described herein relate generally to portable electronic devices, and more particularly, to determining a position fix with portable electronic devices.
Relevant Background
Portable electronic devices, referred to herein as a mobile platforms, conventionally use an indoor positioning engine to determine an indoor position fix. One of the key inputs to an indoor positioning engine is the received signal strength indication (RSSI) measurements from access points (APs). The RSSI measurements made by a mobile platform are evaluated against an RSSI heatmap. An RSSI heatmap is essentially a map of the signal strength for a particular AP based on the distance from the location of the AP. Using RSSI measurements from multiple APs and heatmaps for each AP, the position of the mobile platform may be determined. One particular advantage of RSSI measurements is that they may be carried out by a mobile platform without associating with the APs, which saves time, energy and the need for authentication.
The RSSI values are a measure of the strength of a signal transmitted by an AP. Thus, in order to accurately estimate RSSI values for use with an RSSI heatmap, the transmission power of the AP must be known. Conventionally, measured RSSI values are used with heatmaps assuming that the transmit power is non-varying and is either known a priori or assumed to be a standard transmit power (e.g., 17 dBm or 20 dBm).
Certain APs, however, such as Cisco Lightweight APs, vary the transmission power of an AP (referred to as dynamic transmit power control) based on factors such as the AP load, interference from nearby radio frequency (RF) sources, and other similar factors. Thus, the transmission power for APs using dynamic transmit power control cannot be known a priori or assumed to be any particular value. Accordingly, in an environment with APs having dynamic transmit power control, navigation using RSSI heatmaps is conventionally not possible.